In
instant runoff voting, voters rank their choices. If someone wins an
outright majority based on first choices, the election is over.

If
not, then the candidate receiving the least number of first-choice
votes is removed, and his/her supporters have their votes switched to
their second choice.

The assumption is that were an actual runoff
held with one candidate removed, only voters who liked that candidate
would change votes. Other voters would stay with their first
preference. Through rankings, this happens automatically.